The present invention relates to a fixing unit for fixing a toner image on a recording medium in an electrophotography device.
As disclosed in Japanese Patent Laid-Open No. H5(1993)-127551, a fixing unit is proposed that includes a belt looped over a drive roller and a driven roller and a belt looped over a pair of rollers each of which is pressed via the former belt onto each of the drive roller and the driven roller.
When a belt is used for a fixing unit in an electrophotography type image forming device using toner, the width of nip where paper is pinched and heated can be elongated in the carrying direction. For this reason, even if the carrying speed of the paper is increased, toner can be heated for a sufficient time and hence a recording speed can be increased. In a fixing unit using a belt, there is a construction in which nip is defined by a roller and a belt. In this construction, the paper and the belt can be carried by the roller, which can simplify a drive mechanism. However, to widen the width of nip, an angle formed by a belt looped over the roller needs to be increased, which increases the degree of curl of paper along the roll. To decrease this angle to decrease the degree of curl, the diameter of the roller needs to be increased, which increases the size of the fixing unit and increases heat capacity to increase a rise time that elapses until the belt reaches a fixable temperature. In the case where both parts to define the nip are belts, the problem of curl can be eliminated but a force that presses toner onto the paper at the nip defined by the belts opposed to each other becomes weak. For this reason, to increase a fixing capability, the width of the nip needs to be further elongated as compared with a case where one of the parts to define the nip is a roller. Further, to drive and carry the belt, at least two rollers for each of the belts, that is, a total of four rollers need to be provided. This makes the construction of the fixing unit complex and elongates a rise time that elapses until the belt reaches a fixable temperature.